Is it a helicopter? I know they run cheaper than most transport options for long distances. The hospital should have a case manager or social worker who can help you, do you have the option to ask for their help? They can usually determine the cost up front and see what oop will be with insurance and what not. Good luck
Does the unit/hospital have a case manager or social worker? In my experience, they should be the ones most equip to helping you with this. Can they price comp a plane vs a helicopter? Can they contact his normal insurance to see if they can help with the oop cost in addition to what the travel insurance will pay? I'm sorry you're going though this.
Call his travel insurance co first. It is supposed to cover to and from the destination just not while he is on the ship. If you can't get anywhere with them then call his insurance and see if they offer any type of coverage. That is where I would start.
Do you know what hospital he'll be at here? If you've had contact with them to arrange transfer I would try to talk to their social worker about recommendations.